HMW-PE and UHMW-PE powders are typically produced in slurry by using a Ziegler-type process. General features of HMW-PE and UHMW-PE production processes are disclosed in several patent publications. For example, U.S. Pat. No. 6,486,270 discusses preparation of a high molecular weight polyethylene. Manufacturing processes for the production of UHMW-PE are disclosed by U.S. Pat. No. 5,587,440 and EP 645,403. Catalysts with high activity for these processes have been recently developed. One example of such a catalyst system is the reaction product of titanium tetrachloride and trialkylaluminum.
A certain amount of residual catalyst byproducts, e.g., chloride ion, invariably leaches from the catalyst and remain in the HMW-PE and/or UHMW-PE after polymerization. In the presence of water, these residual catalyst byproducts can create chlorine and hydrochloric acid, which can potentially damage or corrode the equipment used during the polymer processing.
In order to reduce the potential for corrosion, chlorine/acid acceptors or scavengers are generally added in low levels, typically about 0.01-5.00% by weight, to the dry polymer during polymerization or after formation. The acid scavengers most widely used by HMW-PE and UHMW-PE manufacturers are metal soaps. The most common metal soaps used as acid scavengers are stearates, e.g., calcium stearate and zinc stearate. In addition to serving as acid scavengers, stearates also function as internal lubricants and as mold release agents.
Articles formed from HMW-PE and UHMW-PE polymers can be prepared in a one-step process by using high temperature compression, or in a two-step process comprising cold compaction molding followed by high temperature compression molding. During high-temperature compression molding, HMW-PE or UHMW-PE powder is poured into a positive pressure mold that is heated and then cooled under pressure. The cooled mold is then opened to yield a fully sintered HMW-PE or UHMW-PE article. An example of high temperature molding of synthetic resins is provided by U.S. Pat. No. 6,313,208. In accordance with this publication, particles of a highly purified form of hydrotalcite are mixed with a thermoplastic resin for use as a heat stabilizer, or as an acid-acceptor, in a thermal molding process. Although U.S. Pat. No. 6,313,208 discloses HMW-PE and UHMW-PE as examples of thermoplastic resins that may be used in thermal molding, none of the working examples of U.S. Pat. No. 6,313,208 are directed to applications using HMW-PE or UHMW-PE.
In contrast to thermal molding as described by U.S. Pat. No. 6,313,208, HMW-PE or UHMW-PE powder is compressed during cold compaction molding without the application of heat to form a preliminary article, sometimes called a preform. Optionally, the preform is subsequently combined or molded with a second material, e.g., rubber or another plastic, and sintered at an elevated temperature and pressure to obtain the final article.
Previously, it was assumed that the low levels of stearates and other organic-based additives added to HMW-PE or UHMW-PE as acid scavengers during processing had no effect on the cold compaction strength of molded articles comprising the HMW-PE or UHMW-PE resin. Cold compaction strength, also termed “green strength”, is an expression known and used in the art to mean the mechanical strength that a compacted powder must have in order to withstand mechanical operations to which it is subjected after pressing and before sintering, without damage to its fine details (McGraw-Hill Dictionary of Scientific and Technical Terms, Second Edition, 1978).
However, Applicants have unexpectedly found that the internal lubricating properties of stearates significantly weaken the bonding properties of HMW-PE and UHMW-PE granules during cold compaction. HMW-PE and UHMW-PE resins containing metal soaps have been found to have a significantly lower compaction strength than pure resins. By pure resin, it is meant to include virgin resin, i.e., additive-free resin that may have residual catalyst byproducts. The metal soap additives lubricate the HMW-PE or UHMW-PE particles during compaction and, consequently, the preform obtained is especially susceptible to crumbling. Furthermore, a small variation in the metal soap concentration in the resin leads to a great variation in the cold compaction strength values of the articles and, therefore, it is especially important to carefully monitor addition of metal soaps to the resin in order to avoid over- or under-addition.
Therefore, it would be desirable to have methods for the preparation of articles formed from cold compacted HMW-PE or UHMW-PE, which do not contain metal soaps, and which do not show significant degradation of cold compaction strength.